Principal applications of thermosetting unsaturated polyester resins include surface coatings and molding compositions which usually contain reinforcements, such as glass fibers. Such molding compositions are especially suited for the production of reinforced plastics in the form of bulk molding compounds and sheet molding compounds. Boat hulls and automobile bodies are examples of how such molding compositions can be used.
Polyester resins are inherently brittle and it is desirable to reduce this property whenever polyester resins are used as a matrix material. It has been the industry practice to add a flexibilizer to the resin composition to reduce its brittleness. However, such practice generally results in a significant reduction in mechanical properties of the resulting products which is unacceptable for many product applications. It is, therefore, desirable to develop modified polyester resin compositions which are toughened for crack and/or impact resistance by incorporating a component which toughens without flexibilizing such compositions and without significantly contributing to the deterioration of thermal or mechanical properties thereof.